Generally, polyalkylene terephthalates represented by polyethylene terephthalate find extensive utility as, for example, in polyester type synthetic fibers but, they have a disadvantage that they are inflammable and, once they catch fire, they permit rapid spread of the fire. To eliminate this disadvantage, it has heretofore been proposed to incorporate organic phosphorus compounds such as phosphoric esters, phosphorous esters and phosphonic esters into the polyalkylene terephthalates or to have, during the preparation of polyalkylene terephthalates, such halogen compounds as tetrachlorophthalic anhydride and tetrabromophthalic anhydride used as part of the raw materials to afford copolymerized polyesters. The true state of affairs, however, is that such methods of the prior art cannot provide sufficient elimination of the aforementioned disadvantage.
In the circumstances described above, need has been felt for provision of polyesters or polyester compositions possessed of high fireproofness.